1. Field of the Invention
This invention relates to a process for controlling disease in plants due to infection with cercosporin-producing strains of Cercospora. 
2. Description of the Prior Art
Species of the fungus genus Cercospora are widespread, airborne pathogens afflicting a wide variety of agronomically important plants and crops. Spores of Cercospora which have landed on a plant germinate on the leaf surface and ultimately enter the leaf. The fungal mycelium then kills leaf cells, causing severe blighting of the leaf tissue.
Toxin production, particularly the production of cercosporin, plays an essential role in the pathogenicity of Cercospora. At least two types of toxin, beticolin and cercosporin, are produced by Cercospora beticola. Beticolin was reported to be produced by C. beticola (Schlōsser, 1961, Phytopathol. Zeitschrift., 44:295-312) and presents a broad-spectrum of cytotoxic effects (Goudet et al., 1999, Biophys. J., 77:3052-3059). The second toxin, cercosporin, is a broad-spectrum photosensitizing perylene quinone toxin that is produced by members of the genus Cercospora and shows broad toxic activity against wide range of organisms that includes fungi (Chung et al., 1999, Mol. Gen. Genetics, 262:382-389), non-host plants, viruses, bacteria, and animal cell cultures. The structure of cercosporin produced by Cercospora kikuchii Gardner was determined independently by Lousberg et al. (1971, J. Chem. Soc. Chem. Commun., 1971:1463-1464) and Yamazaki et al. (1972, Agric. Biol. Chem., Volume 36, 1707-1718), as 1, 12-bis(2-hydroxypropyl)-2,11 dimethoxy-6,7-methylenedioxy-4,9-dihydroxyperylene3, 10-quinone (molecular weight: 534).
Once infection has been initiated, Cercospora produces cercosporin, which in the presence of light, is photoactivated and reacts with oxygen to produce the highly toxic superoxide anion (O2.−) and singlet oxygen (1O2) in the host plant. The singlet oxygen so produced, which is also the predominant species, acts to disrupt cell membranes (Daub, 1982, Plant Physiol., 69:1361-1364). Indeed, cercosporin has been reported to cause peroxidation of membrane fatty acids that leads to rupture of a plant plasma membranes and cell death. Daub and Briggs (1983, Plant Physiol., 71:763-766) suggested that break down of the host plant cell membrane by cercosporin provides Cercospora with nutrients required for growth and sporulation. We observed cercosporin in newly produced hyphal strands from mycelial plugs that were transferred two day earlier to fresh PDA plates. Observation of cercosporin in young hyphae is consistent with its role in obtaining nutrients from the host. For survival, cercosporin must be produced early after infection to obtain nutrient from the host. This observation is consistent with previously reported observations (Daub et al., 1992, Proc. Natl. Acad. Sci. USA, 89:9588-9592).
Leaf spot caused by Cercospora beticola Sacc. is one of the most important diseases of sugar beets (Beta vulgaris L.). The disease has been reported wherever sugar beet is grown (Bleiholder and Weltzien, 1972, Phytopathol. Zeitschrift., 73:93-114) and results in significant root yield loss and reduced sugar content of beets (Shane and Teng, 1992, Plant Dis., 76:812-820; Smith and Ruppel, 1971, Phytopathology, 61:1486-1487). Gross losses reach as high as 30% even in conditions wherein the disease severity is rated at 3 on a scale of 0-10 (Shane and Teng, 1992). Besides cultural practices and use of resistant varieties, control of Cercospora leaf spot has relied significantly on the use of several different fungicides (Windels et al., 1998, Plant Dis., 82:716-725). While applications of fungicides have proved effective in the management Cercospora leaf spot, rapid development of resistance by C. beticola against several of the fungicides has presaged the need for an alternate management strategy, such as biological control. Recently, the isolation of a Dtox gene from cercosporin-resistant microbes and the transformation of plants therewith has been described by Robeson et al. (U.S. Pat. No. 5,262,306). Daub et al. (U.S. Pat. No. 6,063,987) described the transformation of plants with a fungal gene encoding a protein conferring resistance to cercosporin. Upchurch et al. (U.S. Pat. No. 6,077,995) disclosed the production of transformed plants which exhibited increased resistance to cercosporin toxin by making the plant incapable of moving the toxin across its plasma cell membranes.
Several soil-inhabiting fungal agents, including Laetisaria arvalis, have been described for the control of variety of pathogens in different crops (Lartey et al., 1994, Soil Biol. Biochem., 26;81-88; Lartey et al., 1991, J. Phytopathology, 133;89-98). The Basidiomycete L. arvalis was first isolated from sugar beet residues in the soil by Boosalis in Nebraska in 1960 and was initially referred to as Corticium. sensu lato (Burdsall et al., 1980, Mycologia, 72:728-736; Hoch and Fuller, 1977, Arch. Microbiol., 11:207-224; Odvody et al., 1977, Biological control of Rhizoctonia solani, IN: Proc. Am. Phytopathol. Soc.), but was later placed in the genus Laetisaria by Burdsall et al. (Burdsall et al., 1980, ibid). L. arvalis has since been shown to have biocontrol activity over several plant seed and soil-borne pathogens, including *Rhizoctonia solani, Pythium ultimumn, and Phoma species when applied as a seed coating or as a soil amendment. L. arvalis has been examined for protection of a variety of agronomically important crops from these soil pathogens, including sugar beets, table beets, cucumbers, beans, soybeans, and turfgrass, by Hoch and Abawi (1979, ibid), Lewis and Papavizas (1980, Phytopathology, 70:85-89), Odvody et al. (1980, Phytopathology, 70:655-658), Allen et al. (1982, Mycol. Soc. Am. Newsletter, 33:34), Larsen et al. (1985, Plant Dis., 69:347-350), Martin et al. (1984, Phytopathology, 74:1092-1096), Conway et al. (1997, Plant Dis., 81), Gupta et al. (1999, J. Phytopathology, 147:19-24), and Conway et al. (2000, Proc. Okla. Acad. Sci., 80:39-46).
However, despite these advances, there is a persistent need for improved biocontrol agents for protecting plants from the effects of infection with Cercospora. 